The majority means of hydrogen production today uses steam methane reforming of natural gas (SMR). In North America and in particular Canada, the demand for hydrogen is expected to increase at rates significantly higher than general economic growth because of demands within the petroleum industry, resulting from both the increase in demand from domestic oil supplies as well as the increased need for hydrogen to refine heavier crude oils. At the same time as this demand increase is occurring, “conventional” sources of natural gas in North America are being depleted, tightening the supply of gas and raising gas prices. The increase in demand for hydrogen and reduction in domestic reserves is seen as a long-term trend and is feeding the search for alternative processes of hydrogen production.
The leading alternative to SMR is thermal gasification of carbonaceous materials such as refinery residuals (petroleum coke) or coal by partial steam oxidation, which uses heat, and an oxidant which can be pure oxygen, air and or steam to produce a synthesis gas consisting of CO, CO2 and hydrogen, and depending on the carbon source impurities other gaseous impurities. Gasification processes are well developed in the power industry where the output from gasification is used as a fuel (“BTU”) gas to generate electricity such as by a combined cycle gas turbine. If hydrogen production is the priority rather than producing a combustible fuel gas mixture of CO and H2, a second step uses steam and a water shift reactor to convert the CO to hydrogen and CO2 thereby increasing the hydrogen production for unit input of coal or carbon feedstock.
The amount of hydrogen produced depends on the hydrogen content in the feed materials, which determines the amount of hydrogen produced in the gasification step; and the heat content of the feed materials (and amount of oxygen available), which in turn determines the net rate of CO production in the gasification step and hence the amount of hydrogen produced in the water shift process. In the water shift reactor the hydrogen is produced coincidentally with CO2 and so a gas separation process such as pressure swing absorption or amine scrubbing is needed to separate the CO2 from the hydrogen.
A draw back of the thermal gasification process is that the combustion needs to be carefully controlled to insure heat balance and efficiency, and hence the process favors carbon materials with relatively high heat content. Also, as the moisture and ash levels in the carbon material increase, the amount of oxygen needed for combustion increases, more CO is converted to CO2 to provide heat, reducing hydrogen recovery in a hydrogen priority process and yielding a carbon off gas having a lower heating value or BTU content while at the same time increasing the size of the reactor needed. The gas separation process required to extract hydrogen from the output of the gasifier, which is a mixture mostly of CO2, CO and hydrogen, is energy intensive and is a significant adder to the plant cost. The interaction of these variables can result in a variation in hydrogen production rate, resulting in the need for process buffers and storage to average out these variations when connected to down stream processes, making it less suitable than SMR for the controlled delivery of hydrogen particularly for very large hydrogen demands.
The use of steam oxidation of iron is a well-known process for producing hydrogen. The earliest references to steam-iron processes can be traced back to early experiments to isolate hydrogen (Lavoisier 1783) and during the period of the French Revolution when steam oxidation of hot iron filings was used to produce hydrogen as a lifting gas for military dirigibles. Typically a reducing gas, CO and hydrogen, generated from gasification of a carbon feedstock such as coal or wood (char) was used to return the iron oxide to the iron state. Process improvements were introduced where a sequence of reactors operating at different points of the reduction-steam oxidation cycle were set up so as to generate a quasi-continuous hydrogen production rate, such a process was proposed in the U.S. Pat. No. 1,345,905 issued to Abbott (1920).
Combining reduction and steam oxidation in a single reactor to achieve a continuous steam iron process has been proposed for reactants in a solid state (U.S. Pat. No. 3,619,142 (1971) issued to Johnson et al), where the reduction reaction occurs in a fluidized bed of iron oxide and solid carbonaceous materials.
U.S. Pat. No. 4,555,249 (1985) issued to Leas et al. discloses a reactor design using powder iron and iron oxide wherein the density difference is used to separate the material in the two zones, one for steam oxidation and one for reduction. The difficulty with these processes is the rate of reaction and hence hydrogen production rate is very low. Also the steam oxidation of solid iron is a surface reaction; hence the volumetric efficiency of the reactor is low which is a drawback particularly for large production capacities. Controlling the rate of hydrogen production is also problematic for powder systems as it is difficult to achieve a consistent steam oxidation rate in a large fixed bed reactor because steam-oxidation, and hence hydrogen production, occurs at the surface, and the amount of reactant surface is changing and hence the amount of steam needed to achieve a certain hydrogen production rate changes. To achieve a constant rate of hydrogen production a highly variable steam source or a process to remove oxidized iron powder and add new iron is needed.
The issues of low rate of reaction and low surface area, occurring when solid iron filings are used, can be overcome by using molten iron. Earlier processes for decomposition of methane to hydrogen are described where the carbon dissolved into the iron is released by blowing oxygen, see U.S. Pat. No. 1,803,221, (1931) issued to Tyrer. The process of solid carbon injection in molten iron follows from the experience with iron bath smelting and reduction processes such as Hlsmelt (Hlsmelt Pty Ltd, Australia, 1982). Various processes for gasification of solid carbon materials using a molten iron bath have been proposed.
For example U.S. Pat. No. 4,406,666 (1983) issued to Pashen et al. describes a continuous reactor, which involves a molten iron bath and injection of carbonaceous material and oxygen where the carbon material, oxidizing materials and slag forming materials are added below the surface of the melt. In one instance a two-chamber reactor is proposed in which in one reactor carbon, slag forming materials and oxidant is injected into iron to gasify carbon and produce a syn gas composed of hydrogen and CO. The slag is pushed out of the top of the reactor and desulphurized in the second chamber by injection of an oxidant.
U.S. Pat. No. 4,389,246 (1983) issued to Okamura et al. describes a process for injecting oxygen and steam and coal into a single chamber molten metal bath to produce syn gas using non-submerged lances positioned above the molten iron bath. A stirring gas is injected in the bottom of the reactor to stir the molten iron bath. By maintaining a certain geometry and velocity the reaction is contained. As a consequence a minimal amount of the material is ejected from the bath resulting in less slag sticking to the walls of the containment vessel above the molten metal bath, which can result in a constriction impeding materials flowing in and out of the reactor.
U.S. Pat. No. 6,350,289 (2002) issued to Holcombe et al describes various processes for the extraction of hydrogen from coal and carbonaceous materials using molten iron baths. In this case the gasification process produces hydrogen when carbon feeds are injected into molten iron. In a second step oxygen is injected to decarbonise the iron and provide heat to the bath. Typically mixtures of materials are used with a component being a hydrogen rich component or high in hydrogen content such as methane. In this system the iron is not oxidized and instead the process is controlled such that the carbon content in the iron is maintained above a specified limit, in a well-mixed system.
In implementing the process a two-stage process is proposed. In the first stage the carbon material is dissolved into the molten iron and hydrogen is released from the carbon. In the second stage an oxidizing gas is used to release the carbon in the iron. In the steam oxidation reaction the equilibrium oxygen concentration is controlled below the level that a separate iron oxide phase would form. Although this is a very efficient process for gasifying carbon materials to produce a fuel gas, the amount of hydrogen produced depends on the hydrogen content of the carbonaceous feed and hence favours carbonaceous materials such as methane and ethane having a high H:C ratio. The purity could be an issue.
Methods for continuous de-slagging of molten iron reactors are described in U.S. Pat. No. 4,559,062 issued to Hiraoka et al (1984) which involve the use of pressure control valves to create a pressure difference between compartments in a multi-chamber reactor to push slag out of the reactor. In another case the reactor is rotated to move the molten iron from one compartment to another, see U.S. Pat. No. 4,406,666 (1983) issued to Paschen, and in another case gas lift is used to generate circulation in a molten iron loop, see U.S. Pat. No. 4,338,096 (1982), issued to Mayes.
Therefore, there is a need for an economical method of continuously producing hydrogen of high purity at a controlled rate.